1. Field of the Invention
The present invention relates to a positively chargeable toner for use in electrophotography.
2. Discussion of Background
Varieties of electrophotographic methods have been disclosed, for example, in U.S. Pat. No. 2,297,691 and Japanese Patent Publications 49-23910 and 43-24748. Generally, such electrophotographic methods are capable of providing copies of images by the following steps: Electrostatic latent images are formed on a photoconductor which comprises an electrophotographic material by a variety of methods, the electrostatic latent images are developed to visible toner images with toner, and the visible toner images are transferred to a transfer sheet such as a sheet of paper and fixed thereto by the application of heat and/or pressure thereto, whereby copies of the images are obtained.
The methods of developing such electrostatic latent images can be roughly classified into a liquid development method and a dry development method.
In the liquid development method, there is employed a liquid developer in which finely-divided particles of a variety of pigments or dyes are dispersed in an insulating organic solvent.
In the dry development method, a toner comprising a natural or synthetic resin and a coloring agent, such as carbon black, which is dispersed in the resin, is employed, for example, as in the conventionally known cascade method, magnetic brush method and powder cloud method. A specific example of the dry development method is a method using a two-component dry developer.
Such a two-component dry developer comprises relatively large carrier particles and small toner particles which are held on the surface of the carrier particles by the electrostatic force generated by the friction between the carrier particles and the toner particles.
When such a two-component dry developer is brought near electrostatic latent images, the toner particles are attracted to the electrostatic latent images by the electric field formed by the electrostatic latent images and are caused to be deposited on the electrostatic latent images, with the bonding force between the toner particles and the carrier particles being overcome by the electrostatic attraction between the toner particles and the electrostatic latent images, so that the electrostatic latent images are developed to visible toner images.
Therefore, it is required that the carrier particles be capable of triboelectrically charging toner particles to the desired polarity with a sufficiently large charge quantity for developing electrostatic latent images for an extended period of time for use in practice.
However, in the case of conventional developers, a toner film is formed on the surface of carrier particles while in use by the mechanical collision between the toner particles and the carrier particles or between these particles and parts of a development unit, for example, in a copying machine, and/or by the heat generated by the above-mentioned mechanical collision.
The phenomenon that such a toner film is formed on the surface of carrier particles while in use is generally referred to as "spent phenomenon". Once such a spent phenomenon takes place in the developer, the charging characteristics of the carrier particles are significantly degraded with time while in use, so that it is required that the developer be replaced with a new developer in its entirety.
Various methods of preventing the occurrence of such a spent phenomenon have been conventionally proposed by coating the surface of carrier particles with a variety of resins.
In one of such conventional methods, a silicone resin is coated on the surface of carrier particles to prevent the occurrence of the spent phenomenon. This method, however, has the shortcoming that the silicone-resin-coated carrier particles cannot triboelectrically charge toner particles sufficiently for use in practice.
In order to triboelectrically charge toner sufficiently, there are conventionally known, for example, a method of utilizing the triboelectric charging properties of a binder resin which is employed as the main component of the toner, and a method of adding a charge-imparting agent to the toner.
Specific examples of a positive charge imparting agent which are used in practice are Nigrosine dyes as disclosed in Japanese Patent Publication 41-2427, and quaternary ammonium salts as disclosed in U.S. Pat. No. 3,565,654, and Japanese Laid-Open Patent Application 60-169857.
Furthermore, Japanese Patent Publication 59-9906 discloses a positively chargeable toner comprising a laked xanthene dye.
Conventional positive-charge imparting agents have the shortcoming that the charge quantity of the developer in which the positive-charge imparting agents are employed is largely changed depending upon the ambient conditions.
Recently it is required that the particle size of toner be significantly reduced in accordance with the demand for fine toner images with high quality. For producing such finely-divided toner particles, it is required that a charge-imparting agent be extremely uniformly dispersed in each toner particle. Conventional charge imparting agents, however, cannot meet this requirement, so that improper deposition of toner particles on the background of images, and scattering of toner particles occur when used in practice.
A quaternary ammonium salt disclosed in Japanese Laid-Open Patent Application 63-60458 is a charge imparting agent which is improved with respect to most of the above-mentioned shortcomings of the conventional charge imparting agents.
This charge imparting agent, however, cannot solve the conventional problems sufficiently when used in combination with a developer which comprises silicone resin coated carrier particles and extremely finely-divided toner particles, so that the toner deposition on the background of images and scattering of the toner particles still occur when the charge imparting agent is employed.